1 Answer
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The page range formation is disabled via a command line option, rather than an instruction in the .ist file. So the solution is to call makeindex with the -r option; with imakeidx this can be specified in the value for the option key; the simplest call would be

\makeindex[options= -r]

if no other setting for the (main) index is needed. In your case, denoting with ... other keywords or command line options, something like

\makeindex[... , options= -r -s myindex.ist ... , ...]

Note This is imakeidx specific. If one doesn't use this package, the call from the command line should be